Umm, hydraulic pistons have two important characteristics: force and stroke. Two identical cylinders with equal pressures placed in parallel double force, the same two in series double stroke. Sort of the same way identical batteries in parallel double current, the same in series double voltage. Likewise adding links to a chain makes it longer, not stronger. The discussion was about stationary brake calipers with opposed pistons--effectively hydraulic cylinders in series. So you would get a potential increase in stroke, but no increase in force. But since a fixed brake caliper needs no more total stroke than a floating caliper it's more comparable to replacing one long cylinder with two short ones stacked on top of each other. Someone did mention earlier in the discussion a caliper with parallel cylinders. He also mentioned that the cylinders are reduced in size--that keeps down the force on the pads and reduces the amount of fluid the master cylinder would need to move to apply the brakes. But that's a different arrangement. You mentioned two bottle jacks, but did not say if they were in series or parallel. So, yeah, under the right circumstances two, ten, or a hundred one ton bottle jacks push no harder than a single one ton jack. The formula is more correctly written as Force = piston cross sectional area X pressure - friction. Pad psi = force / pad cross sectional area. You'll note that pads follow the same rule: two pads in series have the same psi as one pad with the same area. But you get more friction since friction force = total pad area X pad psi.